Electronic power supply device

ABSTRACT

A power factor correction circuit using a first rectifier diode to control the series charging of two capacitors and two rectifier diodes to control the parallel discharging of the two capacitors. The power factor correction circuit further includes a resistor that is series-connected with the first diode to improve the power factor and to reduce the current drawn when the voltage is turned on. The rectifier diodes are advantageously voltage limiting diodes to protect the downline circuitry against overvoltages. In one improvement, a current-controlled electronic switch is used for protection against overvoltages that are just above the peak value of the line voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electronic supply device. More particularly,it relates to a device with a power factor correction circuit.

2. Description of the Prior Art

There is a known circuit for the correction of power factor without lineinductance. The circuit is connected to the output of a bridge rectifierand uses rectifier diodes for the series charging and paralleldischarging of two filtering capacitors. An electrical diagram of such acircuit is shown in FIG. 1. A bridge rectifier 1 receives a periodicallyvarying voltage V_(AC) at a pair of input terminals. A power factorcorrection circuit 3 is connected between the rectifier 1 and a load 2at a pair of output terminals of the rectifier. This circuit 3 uses twofiltering capacitors, C1 and C2, having the same capacitance.

A first rectifier diode D1 is connected directly between the twocapacitors C1 and C2. The assembly is connected between two outputterminals of the bridge rectifier.

A second rectifier diode D2 is reverse-connected in parallel with thecombination of the first capacitor C1 in series with the first diode D1.

A third rectifier diode D3 is reverse-connected in parallel with thecombination of the second capacitor in series with the first diode D1.

The working of such a circuit shall now be explained with reference tothe curves shown in FIG. 2.

In a steady operating state, at a start of a half-wave of the linevoltage V_(in), the diode D1 is off. The diodes D2 and D3 are on. Thiscorresponds to the end of the period of the discharging of thecapacitors C1 and C2. When the line voltage V_(in) exceeds the chargingvoltage of the capacitors, the diodes D2 and D3 go to the off state.Then, when the line voltage V_(in) exceeds the sum of the chargingvoltages of the two capacitors (Vc1+Vc2), the diode D1 becomesconductive (T1) and the two capacitors are charged in series until theline voltage reaches its peak value Vc (T2). The diode D1 then goes backto the off state. The two capacitors are each charged at Vc/2 (beingidentical capacitors).

The line voltage, which then decreases, becomes lower than this chargingvoltage Vc/2: the diodes D2 and D3 therefore come on, while D1 remainsoff (T3). The capacitors are again parallel-connected. The capacitor C1supplies the load through the diode D3 and the capacitor C2 supplies theload through the diode D2. This process stops as soon as the linevoltage V_(in) again starts increasing (at the next half-wave) andbecomes greater than the voltage of each capacitor: the diodes D2 and D3go back to the off state, the diode D1 remains off. The system is thenat T0, and the cycle then repeats. The current waveform I_(in) shown inFIG. 2 is obtained.

Between T0 and T1, it is the mains supply system (V_(AC)) that directlysupplies the load (with D1, D2 and D3 off). The shape of the currentwaveform for a value of power P_(out) consumed in the load 2 is given bythe relationship:

I _(in(t)) =P _(out) /V _(in(t))

For Pout constant, between T0 and T1, V_(in) increases and I_(in)decreases.

Between T1 and T2, the capacitors are charged. On top of the currentconsumed in the load 2 (shown in dashes), there is superimposed thecharging current for the capacitors.

Between T2 and T3, the charging of the capacitors, each at half of thepeak voltage Vc, is over. The current I_(in) is only the currentconsumed in the load 2 and the waveform of the current is given by therelationship:

I _(in(t)) =P _(out) /V _(in(t)).

The line voltage decreases and I_(in) decreases (with P_(out) constant).

Finally, between T3 and T0, it is the capacitors C1 and C2 that supplythe load 2. The current I_(in) drawn from the rectifier is zero.

The circuit 3 therefore makes it possible to increase the angle of flowof the bridge rectifier. The waveform of the current I_(in) is spreadover the voltage half-wave with three phases of conduction: [T0-T1],[T1-T2] and [T2-T3]. In this way, the power factor of the device (namelythe ratio of the actual power to the apparent total power) is improvedsince the line is forced to consume current during the most significantpart of the voltage wave, namely when the instantaneous value of theline voltage exceeds half of the peak value Vc.

However, for the charging of the capacitors, there is a drawing ofcharging current which gives rise to a steep leading edge of the linecurrent. There is therefore a current peak. This corresponds tonon-negligible low frequency harmonic contents that limit the value ofthe power factor (with a supply of power at harmonic frequenciesdifferent from the line frequency).

SUMMARY OF THE INVENTION

An object of the invention is to improve the afore-mentioned powerfactor correction circuit.

An object of the invention is to reduce the low frequency harmoniccontents of the waveform of the current drawn from the rectifier.

As characterized, the invention relates to an electronic supply devicefor a load comprising a bridge rectifier receiving a periodic voltage ata pair of input terminals and a power factor correction circuitconnected to a pair of output terminals of the rectifier. The powercorrection circuit includes two capacitors, a rectifier diode to chargethem in series and two rectifier diodes to discharge them in parallel.According to the invention, the correction circuit further includes aresistor that is series-connected to the first rectifier diode to limitthe current drawn in the capacitors and reduce the low frequencyharmonic contents of the current conducted by the rectifier.

The addition of a resistor in series with the diode that enables thecontrol of the charging of the capacitors in series makes it possible toattenuate the charging current. This results in a more rounded-outwaveform of the line current: the low frequency harmonic contents ofsuch a waveform are highly attenuated. The power factor of this deviceis thus appreciably improved.

Furthermore, when the voltage is turned on, the capacitors are chargedimmediately. However, the resistor, in addition to attenuating lowfrequency harmonic contents, will limit the drawn current which, ifexcessively high, damages the diodes and the capacitors.

In one improvement of the invention, a particular three-diode structureof the power factor correction circuit according to the invention isused to protect the circuitry downline with respect to the rectifieragainst overvoltages on the mains supply system.

According to the invention, zener diodes are used as rectifier diodes.In the event of overvoltage in the mains supply system, the three zenerdiodes are series-connected. The circuitry is therefore protectedagainst overvoltages greater than three zener voltages. Each capacitoris protected against overvoltages greater than two zener voltages.

One variant uses a current-controlled power switch parallel-connectedwith the resistor and the diode which controls the series charging ofthe capacitors. A zener diode is used for each of the two diodes thatcontrols the discharging of the capacitors. In this way, it is possibleto protect the circuitry against overvoltages greater than two zenervoltages and the capacitors against overvoltages greater than one zenervoltage.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention are described in detailin the following description made with reference to the appendeddrawings, in which:

FIG. 1 is an electrical drawing of a power factor correction circuitalready described,

FIG. 2 shows corresponding waveforms of current and line voltage,

FIG. 3 shows a diagram of a power factor correction circuit according tothe invention, and

FIG. 4 shows another diagram of a power factor correction circuitaccording to the invention.

MORE DETAILED DESCRIPTION

FIG. 3 shows a diagram of a power factor correction circuit 3 accordingto the invention, connected to a pair of output terminals of a bridgerectifier supplied by a mains voltage V_(AC).

The rectifier 1 and the power factor correction circuit 3 supply a load2.

The circuit 3 has two filtering capacitors Cl and C2 series-connectedwith the output terminals of the rectifier 1. A resistor R and a firstdirectly mounted diode D4 are series-connected between the twocapacitors C1, C2.

A second diode D5 is reverse-connected and parallel-connected with theseries-connected assembly formed by the first capacitor C1, the resistorR and the first diode D4.

A third diode D6 is reverse-connected and parallel-connected to theseries-connected assembly of the second capacitor C2, the resistor R andthe first diode D4.

The assembly formed by the resistor R and the first diode D4 controlsthe series charging of the capacitors C1 and C2. The resistor Raccording to the invention makes it possible to limit the current drawnin the capacitors when the voltage is first turned on. In the worstpossible case, when the equipment is turned on at a time correspondingto a half-wave peak, the value of the drawn current will thus be limitedto Vc/R, Vc being the peak voltage of the line voltage. The resistor Raccording to the invention further enables the charging current to beattenuated. This results in a rounding out of the waveform portion ofthe corresponding line current as shown in dashes in FIG. 2 (interval[T1-T2]).

The trade-off here is that the resistor consumes current. However, ithas been determined in practice that for a resistor consuming only 1% ofthe power available, the power factor of the device is very appreciablyimproved.

In one example, for 100 watts, with:

V_(AC)=230 volts AC,

C1=C2=40 microfarads,

R=20 ohms, P_(Rmax)=1 watt,

there is obtained a power factor PF=0.88.

Finally, the unit formed by the device for rectifying and power factorcorrection may advantageously take the form of an integrated circuit.

In one improvement shown in FIG. 3, zener diodes are used as rectifierdiodes D4, D5 and D6.

Indeed, the structure of the power factor correction circuit 3 has threerectifier diodes series-connected with the pair of output terminals ofthe rectifier. By using three voltage limitation rectifier diodes, it isthen possible to shield the downline circuitry (the load 2) againstmains overvoltages greater than three times the limitation voltage(three zener voltages). Furthermore, the structure has two rectifierdiodes parallel connected with each of the capacitors.

In the same way, by using two voltage limiting rectifier diodes, eachcapacitor is shielded against overvoltages greater than twice thelimiting voltage (namely two zener voltages). This limiting voltageshould be chosen as to be greater than half the peak value Vc of theline voltage.

One variant of the power factor correction circuit used for theprotection against overvoltages according to the invention is shown inFIG. 4. The diodes D5 and D6 which control the discharging of thecapacitors in parallel are voltage limiting rectifier diodes.

However, in parallel with the first rectifier diode D4 that is in serieswith the resistor R, there is placed an electronic switch T1current-controlled in reverse by one of the zener diodes, D5 in theexample. This electronic switch is, for example, a triac, and a resistorr1 connected to the zener diode D5 gives the negative trigger currentneeded to activate the triac.

In the event of overvoltage, the assembly D6, T1, D5 allows the passageof the reverse current and enables protection against overvoltagesgreater than only twice the zener voltage.

As we have seen here above, the diodes D5 and D6 must be such that theyremain off when the line voltage reaches the peak value Vc. This leadsin practice to taking the following value as a zener voltage:

Vz>Vc/2

For example, for 220 VAC, the peak voltage Vc of the rectified linevoltage V_(in) is equal to 310 volts. If the zener diodes are chosenwith a zener voltage of 180 volts, the circuitry is protected forovervoltages of over 360 volts.

This circuit with current-controlled switch requires another zener diodeD7 connected between the zener diode D5 and between the seriesconnection of D4′ and C2 so that the switch is accurately controlledduring the overvoltage. In practice, a 5-volt zener diode is sufficientfor this task.

Finally, in the two examples of a power factor correction circuit withprotection against the overvoltages, the rectifier device and thecorrection circuit may be made in one and the same integrated circuit.

What is claimed is:
 1. An electronic supply device for a loadcomprising: a bridge rectifier having a pair of input terminals thatreceive a periodic voltage and a pair of output terminals; and a powerfactor correction circuit connected to the pair of output terminals ofthe bridge rectifier, the power factor correction circuit including twocapacitors, a first rectifier diode coupled to the two capacitors tocharge the two capacitors in series, and second and third rectifierdiodes coupled to the two capacitors to discharge the two capacitors inparallel, wherein the power factor correction circuit further includes:a resistor that is series-connected with the first rectifier diode tolimit the current drawn in the two capacitors and to reduce lowfrequency harmonic contents of a current conducted by the bridgerectifier; a current-controlled electronic switch that isparallel-connected with the first rectifier diode in series with theresistor; and wherein, to discharge the two capacitors in parallel, thesecond and third rectifier diodes are zener diodes, to protect the loadagainst overvoltages.
 2. The electronic supply device of claim 1,wherein said current-controlled electronic switch is a triac.
 3. Theelectronic supply device of claim 1, wherein a current that controls thecurrent controlled-electronic switch is provided by one of the zenerdiodes.
 4. The electronic supply device of claim 1, wherein the resistoris calibrated to dissipate at most 1% of a nominal power available atthe pair of output terminals of the bridge rectifier.
 5. The electronicsupply device of claim 1, wherein the second and third rectifier diodesare arranged to dissipate overvoltage conditions occurring between thepair of output terminals of the bridge rectifier, and wherein theelectronic supply device is integrated within a single integratedcircuit.
 6. A power factor correction circuit comprising: a pair ofterminals that receive a periodically varying input voltage, the pair ofterminals including a first terminal and a second terminal; a firstcapacitor and a second capacitor coupled in series between the firstterminal and the second terminal, the first capacitor storing a firstvoltage, and the second capacitor storing a second voltage; a firstvoltage controlled switch coupled in series between the first capacitorand the second capacitor that allows charging the first and secondcapacitors when the input voltage is greater than a sum of the firstvoltage and the second voltage; a second voltage controlled switchhaving a first end that is connected to the first terminal and a secondend that is coupled between the first voltage controlled switch and thesecond capacitor the second voltage controlled switch allowing dischargeof the second capacitor when the input voltage is less than the secondvoltage; a third voltage controlled switch having a first end that isconnected to the second terminal and a second end that is connectedbetween the first voltage controlled switch and the first capacitor, thethird voltage controlled switch allowing discharge of the firstcapacitor when the input voltage is less than the first voltage; and aresistor connected in series between the first capacitor and the firstvoltage controlled switch that limits a charging current of the firstand second capacitors and attenuates low frequency harmonic contents ofthe charging current when the input voltage is greater than the sum ofthe first voltage and the second voltage; wherein the second and thirdvoltage controlled switches have second and third overvoltage limitsrespectively, and the power factor correction circuit further comprises:an overvoltage protection assembly having a first end that is connectedbetween the first capacitor and the resistor and having a second endthat is connected to the second end of the second voltage controlledswitch the overvoltage protection assembly limiting a voltage differencebetween the first and second terminals to a sum of the overvoltagelimits of the second and third voltage controlled switches; and whereinthe overvoltage protection assembly includes: a current controlledelectronic switch; and a second resistor, the second resistor beingconnected in series with the current controlled electronic switch. 7.The power factor correction circuit of claim 6, wherein the power factorcollection circuit further comprises: a fourth voltage controlled switchthat couples the second end of the second voltage controlled switchbetween the first voltage controlled switch and the second capacitor toaccurately control the current controlled electronic switch when theperiodically varying input voltage exceeds a sum of the second and thirdovervoltage limits.
 8. The power factor correction circuit of claim 7,wherein: the current controlled electronic switch is a triac; the secondand third voltage controlled switches are zener diodes each having asubstantially similar overvoltage limit that is greater than either ofthe first voltage and the second voltage and lower than the sum of thefirst voltage and the second voltage; and the fourth voltage controlledswitch is a zener diode.
 9. The power factor correction circuit of claim6 or 7, wherein the power factor correction circuit is integrated withina single integrated circuit.